Redesigning the index access method

The old index method is legacy from riak-python-client. This should makes everything simple and slightly magical.

Trying to keep the code as clean as possible.

The basics of the redesign is instead of `add_index` and all that silly stuff, the `Property` will get an additional keyword argument named `index`. If this is set to `True`, indexing will be done automatically. Supported types: `StringProperty`, `NumberProperty`, `ListProperty`, and `ReferenceProperty`